In electrophotography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element comprising at least an insulative photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
In latent image formation the imagewise discharge is brought about by the radiation-induced creation of electron/hole pairs, which are generated by a material (often referred to as a charge-generation or photoconductive material) in the electrophotographic element in response to exposure to the imagewise actinic radiation. Depending upon the polarity of the initially uniform electrostatic potential and the type of materials included in the electrophotographic element, either the holes or the electrons that have been generated migrate toward the charged surface of the element in the exposed areas and thereby cause the imagewise discharge of the initial potential. What remains is a non-uniform potential constituting the electrostatic latent image.
Most electrophotographic elements currently in use are designed to be initially charged with a negative polarity. Such elements contain material which facilitates the migration of positive holes toward the negatively charged surface in imagewise exposed areas in order to cause imagewise discharge. Such material is often referred to as a hole-transport agent. In elements of that type a positively charged toner material is then used to develop the remaining imagewise unexposed portions of the negative polarity potential (i.e., the latent image) into a toner image. Because of the wide use of negatively charging elements, considerable numbers and types of positively charging toners have been fashioned and are available for use in electrographic developers. Conversely, relatively few high quality negatively charging toners are available.
However, for some applications of electrophotography it is more desirable to be able to develop the surface areas of the element that have been imagewise exposed to actinic radiation, rather than those that remain imagewise unexposed. For example, in laser printing of alphanumeric characters it is more desirable to be able to expose the relatively small percentage of surface area that will actually be developed to form visible alphanumeric toner images, rather than waste energy exposing the relatively large percentage of surface area that will constitute undeveloped background portions of the final image. In order to accomplish this while still employing widely available high quality positively charging toners, it is necessary to use an electrophotographic element that is designed to be positively charged. Thus, positive toner can then be used to develop the exposed surface areas (which will have relatively negative electrostatic potential after exposure and discharge, compared to the unexposed areas, where the initial positive potential will remain).
An electrophotographic element designed to be initially positively charged should, however, contain an adequate electron-transport agent (i.e., a material which adequately facilitates the migration of photogenerated electrons toward the positively charged insulative element surface). Unfortunately (and analogous to the situation with positive and negative toners), many materials having good hole-transport properties have been fashioned for use in electrophotographic elements, but relatively few materials are known to provide good electron-transport properties in electrophotographic elements.
A number of chemical compounds having electron-transport properties are described, for example, in U.S. Pat. Nos. 4,175,960; 4,514,481; 4,474,865; 4,559,287; 4,606,861; and 4,609,602 and in Japanese published patent application No. 62-32465. However, many prior art compounds have one or more drawbacks.
Some prior art electron-transport agents do not perform the electron-transporting function very well, especially under certain conditions or when included in certain types of electrophotographic elements. Also, some cause an undesirably high rate of discharge of the electrophotographic element before it is exposed to actinic radiation (often referred to as high dark decay).
Some prior art electron-transport compounds are not soluble or dispersible or have relatively limited solubility or dispersibility in coating solvents of choice or in some polymeric binders desired to be used in charge-transport layers, such that attempts to include sufficient amounts of such electron-transport agents in electrophotographic elements result in some crystallization of the electron-transport agents, which in turn causes problems such as undesirable levels of dark decay and such as unwanted scatter or absorption of actinic radiation intended to pass undisturbed through the charge-transport layer to a radiation-sensitive portion of the element.
Furthermore, some electron-transport agents appear to impart undesirably poor regeneration properties to electrophotographic elements desired to be reusable. Reusable elements are those that can be practically utilized through a plurality (preferably a large number) of cycles of uniform charging (i.e., formation of the initially uniform electrostatic potential), imagewise exposure to actinic radiation to form the electrostatic latent image, and erasure of remaining potential, without unacceptable changes in their performance. Undesirably poor regeneration properties are manifested as a progressive rise of the final potential (also referred to as the residual potential) to which the element can be driven by the erasure process, caused by a buildup of residual charge within the electrophotographic element over time as the element is exercised through its normal cycles of electrophotographic operation. This buildup of residual charge is not removed by normal methods of erasure during normal cycles of operation, such as by exposure to excess amounts of actinic radiation. The resulting unerasable residual potential can build up to a level (e.g., higher than 100 volts) such that the element can no longer be discharged to the intended degree (e.g., from an initial potential of 500 volts to an intended potential of 100 volts) in areas of maximum imagewise exposure during latent image formation. This results in image artifacts such as lower image density in areas of maximum imagewise exposure that are intended to produce maximum image density. In effect, the element has become no longer reusable.
Also, some electron-transport agents suffer from being obtainable only through difficult, lengthy, and/or otherwise relatively inefficient or uneconomical methods of preparation.
Thus, there is a need for electrophotographic elements containing chemical compounds that exhibit good electron-transport properties in the elements without imparting undesirably poor regeneration or dark decay properties thereto. The electron-transport agents incorporated in the elements should be sufficiently soluble or dispersible in coating solvents and polymeric binders of choice, and should be capable of being readily prepared by relatively simple and efficient methods.